Background information on the safety of varicella (chickenpox) vaccines when given in pregnancy

1. What is the risk if you are given varicella vaccine whilst you are pregnant or shortly before falling pregnant?

There is no known risk if you are immunised with varicella vaccine whilst pregnant or shortly before becoming pregnant. Varicella vaccine is not recommended at this time as a matter of caution and it is for the same reason that we wish to follow all women who are given the vaccine by accident whilst pregnant.

A Varicella Vaccine Pregnancy Registry was established in the USA in 1995 to monitor the outcomes of pregnant women who were inadvertently immunised with varicella vaccine 3 months before or any time during pregnancy. Information on the first ten years of the Pregnancy Registry has been published (1).  More recent data are also available: during the first 12 years of the pregnancy registry, there were 628 prospective reports with information available on the pregnancy and baby. No babies were born with congenital varicella syndrome (see below) or birth defects compatible with congenital varicella syndrome. Whilst the number of reports is relatively small, the overall rate of birth defects in women who received varicella vaccine and were reported to the Registry was similar to the expected rate in the rest of the population.

There is no specific safety concern, either for the mother or the baby, when varicella-containing vaccine is given in, or shortly before pregnancy, and no cases of congenital varicella syndrome have been causally linked to varicella vaccine virus. Women who have been immunised with varicella vaccine in pregnancy can therefore be immediately reassured.

In contrast, when a woman has natural varicella infection in pregnancy both the woman and her baby may be affected (see section 3). Rarely this can lead to the baby being born with developmental abnormalities - this is called congenital varicella syndrome (see section 4).

2. Why are you following up women who are given varicella vaccine whilst pregnant or shortly before becoming pregnant?

The Registry detailed in section 1 does not support a relationship between exposure to varicella vaccine during pregnancy and either congenital varicella syndrome or other birth defects, however, the number of women analysed to date is not high enough to rule out a very low risk.

The UK Vaccine in Pregnancy surveillance programme is run by the Immunisation Department of the Health Protection Agency Centre for Infections http://www.hpa.org.uk/. The objectives of the UK Vaccine in Pregnancy surveillance are to compile additional information on women who are immunised with specified vaccines whilst pregnant. These data will be used to help better inform pregnant women who are inadvertently immunised, their families and health professionals who are responsible for their care.

3. What is the risk of natural varicella infection in pregnancy?

If a woman is infected with natural varicella whilst pregnant there are risks to both her and the baby. Varicella can cause severe maternal disease, and 10%-20% of pregnant women infected later in pregnancy develop varicella pneumonia, which can be fatal. Newborn babies whose mothers develop varicella rash from 5 days before to 2 days after delivery are at risk of neonatal varicella, and around 30% of these babies die. Infection of the foetus may result in stillbirth, herpes zoster during infancy or early childhood, or congenital varicella syndrome (see section 4). In contrast, the vaccine contains a weakened strain of the varicella virus and has not been found to cause these problems.

4. What is congenital varicella syndrome?

Babies born to mothers who were infected with varicella during the first half of their pregnancy are at risk for a very serious condition known as congenital varicella syndrome. This is characterised by low birthweight, scarring of the skin, withered limbs, small head, cataracts, and other problems. In a prospective study conducted in Europe from 1980-1993 involving nearly 1400 mothers who had varicella during pregnancy, the highest risk (2%) for congenital varicella syndrome was observed when maternal infection occurred during 13-20 weeks' gestation. Before 13 weeks the risk was estimated to be around 1%.

5. Who are you following up?

We are following up women who have received varicella vaccine from 90 days prior to their last menstrual period or at any time in pregnancy.

(1) Wilson E, Goss M, Marin M et al. Varicella vaccine exposure during pregnancy: data from the first ten years of the pregnancy registry. Journal of Infectious Disease 2008 Mar 1;197 Suppl 2:S178-84.

Last reviewed: 22 January 2009